1. Field of the Invention
The present invention relates to methods and apparatus for demulsifying oil in water by dilution and impact, and for separation of the oil and suspended solids by dissolved gas floatation. Saltwater brought to the surface with gas and oil from underground production wells is commonly referred to as “produced water.” The methods and apparatus of the present invention are particularly suitable for demulsifying oil in produced water using dissolved methane or dissolved carbon dioxide as a diluted and for separating the oil and suspended solids from the produced water also using dissolved methane or dissolved carbon dioxide for floatation.
2. Description of Related Art
The methods and apparatus for dissolving gases in liquids disclosed in patent application Ser. No. 09/897,496, filed Jun. 12, 2001, now U.S. Pat. No. 6,669,843, issued Dec. 30, 2003, in the name of Johnny Amaud and assigned to the same assignee as the present application where liquids can be totally saturated with dissolved gases at any pressure, also provides the opportunity for improvements in the methods and apparatus for separation of both solids and fluids suspended in a second fluid by dissolved gas floatation.
Saturating a liquid as water with a dissolved gas at a high pressure then injecting the saturated liquid into other water contaminated with suspended solids or suspended liquids, such as oil, and releasing the pressure allows the gas to come out of solution as microscopic (small) bubbles that rise slowly in the liquid. As the bubbles form they attach themselves to the contaminating suspended solid particles and liquid droplets increasing the buoyancy and accelerate the rise of the particles and droplets to the surface where they can be removed. The process can be used for separating solids from fluids in dissolved air floatation clarifiers and for separating oil from water using either dissolved air or other gases, such as methane (natural gas) or carbon dioxide. While the demulsification and separation of oil and suspended solids brought to the surface in the production of oil and gas is the exemplary application described in the present invention it is understood that there are many other implementations without deviating from the intent and spirit of the present invention.
The basic requirements of a system used for gas floatation of both solids and liquids have been well defined to include introduction of finely dispersed gas bubbles into the stream to be treated, a floatation zone of minimum turbulence, and a means of removing the floated material. In addition, chemicals to aid in separation of the contaminants from the water are commonly introduced into the stream and have become part of the basic requirements of gas floatation systems. The selection of systems and methods for treating specific wastewater streams typically depend on what contaminants are present, whether the contaminants are predominantly suspended solids with a small amount of liquid contaminants or predominantly liquids with a small amount of suspended solids, and what is to be done with the contaminants removed from the treated water.
Most of the deficiencies of the present dissolved gas floatation systems are in the quality of the bubbles produced, size and cost of the apparatus used to generate the bubbles, method of introducing the bubbles into the stream of water to be treated, and the success of mixing and dispersing the demulsifying agents added by existing apparatus used.
Salt water is typically brought up to the surface with gas and oil from underground production wells. The water is typically referred to as “produced water” and sometimes as “produced brine.” The oil is separated from water by flowing through a high-pressure oil-water separator. The purpose of the oil-water separation is to recover as much of oil as possible. The oil left in the produced water after this stage of separation is primarily emulsified oil mixed with suspended solids.
The produced water then has to be decontaminated for disposal. To remove the emulsified oil produced water may be placed into a tank to act as a static separator. On an offshore production platform, that tank may even be a leg supporting the platform in the ocean. The still produced water allows oil to separate and slowly rise to the surface.
Any additional free oil obtained from this process is added to the oil in production. The emulsified oil will slowly rise to just below the free oil at the top of the tank. Decontaminated water at the bottom of the tank can be removed and disposed of in accordance with the requirements of local, state, federal, and international regulations.
The layer of emulsified oil may sometimes be referred to as the “rag layer.” Understanding what an emulsion comprises is a requirement for possibly recovering additional oil or decontaminating the produced water for disposal.
An emulsion in produced water is a suspension of very finely divided oil droplets dispersed in the water that does not readily separate. Under normal conditions when oil and water come into contact and allowed to stand still they will separate according to specific gravity, with the oil on top of the water. If the oil-water mixture is stirred vigorously, it will take some time for them to separate again. If a small amount of surfactant, such as a dishwashing detergent, is added to the oil-water mixture then agitated, the surfactant will coat each droplet with a thin polarized film that will cause the droplets to repel each other and prevent them from coalescing, and they will remain dispersed. The surfactants are called “emulsifiers.” Certain emulsifiers are naturally occurring in produced water. The emulsified oil droplets may also be mixed with finely coated solid particles from a number of materials to further complicate the emulsion. This occurs when oil with specific gravity lower than water is attached to a particle heavier than water. The combined specific gravity may be equal to that of water and the coated particle may remain in suspension indefinitely to become part of the emulsified oil problem in produced water.
While the problem exists anywhere oil and gas is produced and processed, an offshore platform is not readily accessible and oil and gas production could perhaps be from as many as thirty wells and the quantity of produced water may be enormous. The produced water keeps coming out of the ground; therefore, the first and ultimate objective is cleaning the produced water for disposal. The second objective, of course, is to demulsify and recover as much of the emulsified oil as possible for added profit, preferably without adding chemicals to the oil. The third objective is to dispose of as little of the emulsified oil as possible to reduce costs associated with bringing it onshore for disposal.
Some of the emulsified oil and suspended oil can be separated from the produced water as an entire body by dissolved air floatation, centrifuge, or some other method. The remaining contamination may still exceed that allowed for disposal. Chemical demulsifiers in trace quantities counter the stabilizing forces in the emulsions to allow separation. The cost of using chemical demulsifiers in the large volumes of produced water would be enormous and may outweigh the benefits realized. Dilution with a light hydrocarbon product may be helpful in breaking the emulsion. The use of chemical demulsifiers and diluting agents require thorough mixing to be effective. Mechanical energy can also be used to assist in breaking the emulsion by applying forces greater than the repelling forces between the droplets to bring them together. The dynamics of fluid flow generally can be expressed by conservation of energy, momentum, and impulse. Impulse is the method used to supply the mechanical energy to overcome the repelling forces between the droplets in the present invention. The diluting agents used in the present invention are dissolved methane or dissolved carbon dioxide supplemented by a light petroleum product when it is required.